Vehicle manufacturers are turning away from mild steel sheet panels to aluminum, advanced high strength steel (AHSS), or ultra high strength steel (UHSS) panels. Aluminum, AHSS, and UHSS offer high strength/low weight alternatives to mild steel. These panels, however, have lower ductility than mild steel. One problem found with Aluminum, AHSS and UHSS is cracks developing from the sheared surface of punched holes during dynamic loading.
Due to elevated sensitivity to sheared surface stretchability in AHSS, cracks propagate from the sheared surfaces of the panels during dynamic loading changing the failure mode of the materials. Since mild steels are less sensitive to edge cracking, the failure mode for mild steel panels offered energy absorbing plastic deformation. The failure mode of high strength/low weight alternative material panels do not absorb loading energy as efficiently when cracks are propagated.
Punching is a metal forming process in which material is sheared from a metal member to create a hole. A punch shears a hole in the metal member and the sheared material, also known as a slug, falls away as scrap. In this process, the perimeter of the hole is defined by a sheared surface.
A fastener may be inserted into a hole to join objects together. The fastener has a shaft that passes through the hole of an object to fasten the object to another object. The shaft of the fastener typically abuts the sheared surface of a punched hole.
Sheared surfaces have microscopic cracks. When force is applied to objects that are fastened together with a fastener, the shaft of the fastener may apply stresses to the sheared surface of the hole. The microscopic cracks in the sheared surface of the hole are stress risers that are at locations where tears initiate in the metal member. Sheared surfaces tend to crack, rip, or tear beginning at the cracks due to stresses applied by the fastener.
The following references were considered before filing this application: U.S. Pat. No. 6,968,723 to Akahane et al., U.S. Pat. No. 7,318,696 to Babej et al., U.S. Pat. No. 6,708,542 to Gafri at al., and U.S. Pat. No. 6,631,630 to Pourboghrat et al.